Brake systems comprise not only a service brake but usually also a parking brake in order to be able to shut down the vehicle with the brake engaged. Such a parking brake is also referred to as a handbrake.
Parking brakes for brake systems that are operated by pressurized media generally have spring-loaded brake cylinders in which a storage spring engages the brake so that the vehicle is braked or can no longer be moved. This parking brake is released by applying compressed air to a spring store part of a spring-loaded brake cylinder, so that the storage spring is compressed and the parking brake is therefore released.
The supply of compressed air to the spring store part in order to release the parking brake is provided by means of an air-quantity-boosting valve device, in particular a relay valve. The air-quantity-boosting valve device outputs, at its outlet, the same pressure that is present at its control inlet, but with a boosted quantity of air drawn from a compressed air reservoir.
The control inlet of the air-quantity-boosting valve device is supplied with the control pressure via at least one control valve. This control valve is embodied as an electrically actuatable control valve, wherein the switched state of this control valve is determined by an electrical control device. The control device is in turn electrically connected to an actuation device for actuating the parking brake. This actuation device can be actuated by the driver of the vehicle. The driver can release or engage the parking brake by means of this actuation device.
The spring-loaded brake cylinders usually have a spring store part and a service brake part. The service brake is conventionally actuated by means of a brake pedal, wherein the service brake generally comprises an anti-lock brake device (anti-lock brake system or ABS), which prevents wheels of the vehicle from locking owing to the actuation of the service brake.
In contrast, anti-lock brake devices of the parking brake that prevent wheels of the vehicle from locking owing to the actuation of the parking brake are less widespread. DE 199 42 533 A1 describes a parking brake with such an anti-lock brake device, wherein locking of wheels is prevented on the basis of the wheel speeds detected by sensors. This construction can be problematic in that, when the wheel speed sensors or the electronics that process the sensor signals fail, braking with anti-lock protection is no longer possible when the parking brake is actuated. Furthermore, this embodiment has the disadvantage that costs are incurred as a result of the use of wheel speed sensors and of the cables required for sensors.